Structure of the β-Lactoglobulin Tetramer

Abstract

DETAILED investigations of the association behaviour of the genetic variants of β-lactoglobulin have shown that the A variant aggregates strongly and reversibly to a tetramer^1,2 of 144,000 molecular weight in the cold between pH. 3.7 and 5.2. The thermodynamics of this association have established the tetramer structure to be cyclic, involving the closing of four bonds^1,2, and its hydrodynamic properties^2,3 show it to be compact. A small-angle X-ray scattering examination of this association⁴ has led to the conclusion that, of the various general types of possible tetramer structures, a cubic array of eight spheres was the most probable. This conclusion was based on comparison of the measured radius of gyration of the tetramer (34.4 Å) with values calculated for various models formed of four of the monomeric units deduced by Green and Aschaffenburg⁵ from X-ray diffraction examination. These monomeric units have been shown to persist in solution⁴. The monomer unit consists of a combination of two identical spheres (of molecular weight 18,000) 17.9 Å in radius, impinging by 2.3 Å along their centre-to-centre axis; these two spheres are, furthermore, related by a dyad axis of symmetry through the plane of contact formed between them. Recently, we have carried out a detailed examination of compact models in the light of known structural requirements, the results of which we wish to report in this communication. The basic requirements are that all four monomer molecules must be identical with each other², the monomer structure must be maintained intact in the tetramer⁴, and the structure must fit all known experimental information and be symmetrical in nature⁶.